This invention lies in the field of glass forming compositions having utility in thermally insulating bricks for rotary cement kilns and the like.
The refractory lining of the preheating zone of a rotary cement kiln, that is, the kiln region in which the raw material is dried and calcined, and, depending on whether the method employed involves a wet process, or a dry process, or the process employed utilizes grate or cyclone preheaters, which region constitutes about 25 to 45 percent of the entire length of a rotary kiln, is subject to characteristic stresses which may be described as follows: Since the raw material in this kiln region is in a stage of dehydration and calcination (endotherm processes), temperature stress is secondary as a wear or attrition factor. Mechanical stress as a result of abrasion is moderate, since the raw meal, or the preformed granules, are still soft. Chemical stress as a result of alkali oxides, alkali sulfates, alkali chlorides and alkali fluorides, which alkalis come from the raw material and from the fuel, and which alkalis accumulate in the kiln atmosphere because of the circulation of the kiln exhaust gases for better heat utilization, is, on the other hand, heavy to very heavy. These substances condense, particularly in the region of the preheating zone, on the surface of the refractory brick-lining and diffuse into the brick interior with more or less heavy chemical interaction with the brick components. This has the consequence that, with fireclaybricks of class A of German DIN-classification and corresponding light-weight refractory bricks, which are still used in this kiln region in part, alkali spalling occurs as a result of the formation of feldspar and feldspar-like compositions. A volume increase is associated therewith, and the alteration of the thermal coefficient of expansion associated therewith, in the sense of an increase therein, leads to bursting and rapid brick wear. In part, the open pores also fill up with alkali compounds, as a result of which chipping-off of brick hulls (i.e. outer surfaces) also results.
This realization has already led to the previously used fireclaybricks of class A (Al.sub.2 O.sub.3 content between 30 and 45 percent by weight) being replaced either by fireclaybricks with a low Al.sub.2 O.sub.3 content, or by siliceous light-weight refractory bricks with between 15 and 30 percent by weight of Al.sub.2 O.sub.3. Bricks of this material group demonstrate an improved behavior by comparison to the previously employed fireclaybricks with regard to alkali attack and the compounds thereof, as the indicated reactions do not extend to the entire brick, but rather are characteristically limited only to the surface thereof. As a result of reaction of the alkalis with the brick components, solid protective glaze layers are formed, on the brick surface, which protective layers prevent, or, delay, a deeper in-diffusion of the alkalies into the brick and the above described bursting. This mechanism, in which a "heterogeneous" glaze formation thus takes place on the brick surface, which naturally can only occur if suitable substances are present from the kiln atmosphere which react with the brick material, made possible for the first time by the successful use of porous, thermally insulating bricks, for example, those of the kind described in British Pat. No. 1,005,469, in the preheating zone of rotary cement kilns.
It has shown itself to be the case, however, that the mechanism of superificial "brick sealing" as a consequence of the formation of solid protective layers by "heterogeneous" glaze formation fails to occur, particularly in the presence of KCl or other alkali chlorides, in the following designated simply by "KCl", in a kiln atmosphere. The KCl diffuses substantially completely into the open pores of the brick without reaction with the brick material. Because of the low melting temperature of KCl, and, in particular, the formation of eutectic melts with still lower melting points in the presence of K.sub.2 SO.sub.4, the penetration of the KCl in the case of porous, thermally insulating bricks occurs so deeply that their insulating ability is lost, and the brick structure is destroyed by repeated cycles of solidification and melting, as well as the crystalization pressure of KCl.
In order to prevent infiltration of KCl into the porous, thermally insulating bricks, it is already known that the brick surface can be provided with heat-resistant or refractory protective coatings which contain an inorganic binding agent, such as, for example, water glass, aluminum sulfate, or monoaluminum sulfate. Protective coatings of this kind are, however, only effective for a short time, since they are rapidly worn away (removed) by the kiln feed sliding over them, and the surface of the brick is thus soon re-exposed. This exposed surface is now, in turn, fully exposed to the infiltration of KCl, or the like. In addition, in the case of alkali spalling, brick wear occurs by batch-wise occurrences of peeling-off or chipping-off of layers in a thickness of up to several centimeters, so that even protective coatings with theoretically good adhesion and high abrasion resistance are lost along with the chipped-off brick pieces. Renewing the protective coatings on the brick surfaces can, however, only be undertaken by repeated shutdowns of a rotary kiln, which is completely uneconomical.